This invention relates to compression release engine brakes, and more particularly to devices for controlling the timing of the compression release event in such engine brakes.
Compression release brakes for internal combustion engines are well known as shown, for example, by Custer U.S. Pat. No. 4,398,510. As the '510 patent points out, it is typical to set the starting point for motion of the slave piston in such brakes at a predetermined distance from the exhaust valve surface on which that slave piston operates to allow for thermal expansion of the exhaust valve. Without such a clearance, when the engine became hot, the slave piston would hold open the associated exhaust valve at all times, thereby decreasing the fueling mode power available from the engine and also possibly causing damage to the engine. On the other hand, improved engine brake performance can be achieved by eliminating this clearance (or even going to a negative clearance) during engine braking mode. The '510 patent therefore shows a timing mechanism built into the slave piston return stop screw. This timing mechanism automatically extends from the return stop screw by a predetermined amount during the first cycle of engine brake operation each time the engine brake is turned on. Once extended, the timing mechanism remains in that condition as long as the engine brake is turned on. The extended timing mechanism takes up as much of the original clearance as is desired, and may even extend more than the original clearance in order to provide a negative clearance if that is desired. When the engine brake is turned off, the timing mechanism retracts so that there is no danger of the engine brake holding open the exhaust valves when fueling of the engine resumes.
The timing mechanism shown in the '510 patent is extended by a spring in the timing mechanism. The amount of extension is determined by the dimension of a transverse slot in the timing mechanism through which a pin passes. Once the timing mechanism is extended, it is held extended by hydraulic fluid (typically engine oil being used in the hydraulic circuit of the engine brake) trapped behind the timing mechanism by a ball check valve. When the engine brake is turned off, this trapped oil leaks away and the timing mechanism is pushed back into the slave piston return stop screw by the slave piston return spring.
While the timing mechanism shown in the '510 patent works extremely well, it has some problems and there is room for further improvement. The timing mechanism of the '510 patent has a relatively large number of relatively small precision parts. For example, the '510 patent mechanism has a relatively small plunger which must reciprocate in a bore in the slave piston return stop screw without allowing too much leakage of oil around the plunger. The plunger also requires a seat for the relatively small ball check valve. The ball check valve and an associated spring must be provided. The transverse slot in the plunger must be precisely located and formed in order to ensure the correct amount of extension of the plunger. The transverse pin must be mounted securely to ensure that it cannot come out. A reverse push test may be required to ensure that the pin is in fact secure. All of these considerations make the '510 patent mechanism relatively difficult and expensive to manufacture.
The '510 patent mechanism may also have some shortcomings in use. For example, the slave piston return stop screw may be hollow near the nut which is used to lock the screw in place in the engine brake housing. Over-tightening of this nut may therefore cause the screw to break. The relatively small size of the plunger may cause very large hydraulic pressures to develop inside the screw under some conditions. This may also cause the screw to break just below the above-mentioned nut.
In view of the foregoing, it is an object of this invention to improve and simplify engine brake timing mechanisms of the general type shown in the '510 patent.
It is another object of this invention to provide an engine brake timing mechanism which has fewer parts than the '510 patent mechanism, which is easier and cheaper to manufacture than the '510 patent mechanism, and which is easier and more reliable to use than the '510 patent mechanism.
It is still another object of this invention to provide an engine brake timing mechanism which is not subject to the high internal hydraulic pressure which can occur in the '510 patent mechanism.